high speed misfire
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Joined: Jan 2007
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From: Rocky Hill CT
high speed misfire
I have a 98 A4 with a new LS3 crate engine, and a A&A S.C. I sorted out most of the issues last summer, even running a few traction limited 10 second runs. All of a sudden this spring it idles smooth, and I can easily cruse at 70 or 80 mph but, if I put my foot into it, it coughs and spits, misfires, and actually slows up. Numerous experts told me the first thing to look at were plugs or wires. I replaced the plugs, as well as installing new MSD wires, with no change. Anyone have any ideas? The only item I changed over the winter was a new radiator, and fan assembly.
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Joined: Sep 2013
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From: Cape Coral, Florida
Sorry to hear you're having issues !!...without a scan tool and some other diagnostic equipment it's just a crap shoot...check for DTC's through the DIC to see if you have anything stored...is the MIL illuminated or was it flashing denoting a P0300 random misfire ??...misfires can be fuel, ignition, or even mechanical. Even with new plugs or wires it still can be something electrical in the ignition system or maybe a bad coil...you may have a broken valve spring, burnt valve or a fuel injection problem...without the right diagnostic equipment sometimes it's just better to take the car to a "good" diagnostic shop and pinpoint the problem without you having to shotgun parts that my cost more than just the diagnostic fee !!
Melting Slicks
Joined: Aug 2000
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From: Nashua NH
I just had a similar problem in my truck. Could cruise no problem but as soon as I put my foot down the sputtering began. Pulled the codes and came back with Random Misfire. So I then pulled each wire 1 by 1 until I found the one that made no change in idle. Turned out to be a bad coil over. Replaced the one bad one and been fine ever since.
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I've only ever had this issue once and it was on a Honda, but may help. In my case the car ran great below 3/4 throttle and off the VTEC. Once I put my foot in it and it climbed above 6k it would misfire. In my situation it ended up being a weak coil.
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Joined: Jan 2007
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From: Rocky Hill CT
There are no codes of any kind related to this. There are two responses indicating a bad coil. Even though all the coils are new, it doesn't mean one can't be bad I suppose. Is there a way to check the coils without just replacing all of them? I used NGK, gaped at .35, per A&A. When I had it tuned, the guy I hired required me to buy HP Tuners. I know almost nothing when it comes to computer tuning but, I do have that, if there is some way to use it to find the problem. Thanks for all the responses. If anyone comes up with anything I can use HP Tuners for, let me know. My next step was going to be to hook up my fuel pressure gauge to see if I can find something fuel related.
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Joined: Sep 2013
Posts: 15,496
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From: Cape Coral, Florida
There are no codes of any kind related to this. There are two responses indicating a bad coil. Even though all the coils are new, it doesn't mean one can't be bad I suppose. Is there a way to check the coils without just replacing all of them? I used NGK, gaped at .35, per A&A. When I had it tuned, the guy I hired required me to buy HP Tuners. I know almost nothing when it comes to computer tuning but, I do have that, if there is some way to use it to find the problem. Thanks for all the responses. If anyone comes up with anything I can use HP Tuners for, let me know. My next step was going to be to hook up my fuel pressure gauge to see if I can find something fuel related.
I did a YouTube video a while back on C5 coil functional check...don’t know if it will help !!
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I'm going to assume the ngk tr6, or is it a different plug?... how much boost and are you using pump gas, pump and meth, e85?... since you have hptuners you should set up a datalog config and watch your knock and afr when this is occuring... if your car is speed density tuned it will not throw any misfire codes so that may be why you aren't seeing anything
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From: Rocky Hill CT
The plugs are tr6 per A&A. If my memory is correct, A&A told me it should be around 7% boost, and it is speed density tuned. I run pump gas, as I always have. I am going to record the misfires with HP Tuners, then send it to my tuner. He thinks he might be able to find the problem after looking at the run. I am thinking it is a lean condition, as when I floor it to cause the misfires, I notice that the air/fuel gauge pegs all the way to bright red 18. I replaced the driver's side fuel pump (3rd one) over the winter. That was the only thing changed since last summer, when it ran fine. Perhaps some debris got into the line somehow.
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The plugs are tr6 per A&A. If my memory is correct, A&A told me it should be around 7% boost, and it is speed density tuned. I run pump gas, as I always have. I am going to record the misfires with HP Tuners, then send it to my tuner. He thinks he might be able to find the problem after looking at the run. I am thinking it is a lean condition, as when I floor it to cause the misfires, I notice that the air/fuel gauge pegs all the way to bright red 18. I replaced the driver's side fuel pump (3rd one) over the winter. That was the only thing changed since last summer, when it ran fine. Perhaps some debris got into the line somehow.